1. Field of the Invention
The present invention relates generally to a cylindrical vibration-damping device adapted for use as an automotive suspension bushing, for example, and more particularly to a cylindrical vibration-damping device of fluid-filled type, which is capable of exhibiting vibration damping effects based on the flow action of a fluid filling the interior.
2. Description of the Related Art
There are any number of known cylindrical vibration-damping devices proposed to date for interposition between components that make up a vibration transmission system in order to provide vibration damped linkage or vibration damped support of the components to one another. These cylindrical vibration-damping devices have a construction in which an inner shaft member adapted to be mounted to one component of the vibration transmission system and an outer cylindrical member spaced apart peripherally outward from the inner shaft member are elastically connected with each other by a main rubber elastic body. In an effort to improve vibration damping capabilities of cylindrical vibration-damping devices, there have also been proposed cylindrical vibration-damping devices of fluid-filled type which include a plurality of fluid chambers filled with a non-compressible fluid, and an orifice passage connecting the fluid chambers with one another. Fluid-filled cylindrical vibration-damping devices generally have a construction which includes a pair of fluid chambers opposed to each other in one diametrical direction (which coincides with the principal vibration input direction), and an orifice passage connecting the pair of fluid chambers with one another. On the basis of relative pressure differential arising between the pair of fluid chambers, fluid flow will be produced through the orifice passage and vibration damping action will be attained on the basis of resonance action etc. of the fluid.
In a fluid-filled cylindrical vibration-damping device of this kind, at times of vibration input in the opposing direction of the pair of the fluid chambers, relative pressure fluctuations will be effectively produced between the fluid chambers, and excellent vibration damping action will be attained on the basis of flow action of the fluid. On the other hand, at times of vibration input in other directions such as the direction orthogonal to the opposing direction of the fluid chambers for example, it may suffer from a problem that relative pressure fluctuations between the fluid chambers will not be effectively produced, and therefore enough vibration damping action will not be attained.
There has been proposed, in Japanese Unexamined Patent Publication No. JP-A-3-9137, a fluid-filled cylindrical vibration-damping device that is able to attain effective vibration damping action against each of several types of vibration input in different directions. Specifically, the fluid-filled cylindrical vibration-damping device has four fluid chambers having a prescribed circumferential width and arranged at a uniform angular interval of 90° in the circumferential direction. With this arrangement, effective vibration damping action will be attained against each of vibration inputs in two directions orthogonal to each other.
However, in the fluid-filled cylindrical vibration-damping device disclosed in JP-A-3-9137, since the four fluid chambers having an identical shape with one another are arranged at a uniform interval in the circumferential direction, the springs of two diametrical directions orthogonal to each other are equal. Therefore, in cases where, for example, different vibration damping characteristics are required with respect to two diametrical directions perpendicular to the axis of the device, it was difficult to set different spring constants to different directions and to fully meet required characteristics.
Moreover, since the four fluid chambers having the identical shape with one another are arranged at a uniform interval along the circumference, it was difficult to tune so as to exhibit excellent vibration damping capabilities especially in principal vibration input direction, and sometimes required vibration damping capabilities were not able to be satisfactorily realized.